Mutual inductance in transformer based tank circuitry

ABSTRACT

Placing inductors or resistors in parallel causes the combined value of inductance or resistance to decrease according to the parallel combination rule. This invention decreases the parasitic resistance of an inductor by placing several inductors in parallel. Furthermore, by careful placement of these inductors, the mutual inductance between these inductors can be used to increase the equivalent inductance value to a value near that of the original inductance value of a single inductor. Thus, it is possible to create an inductance with a much lower value of parasitic resistance. This invention allows the formation of high Q inductors and would be beneficial in any circuit design requiring inductances. Another aspect of this invention is that the coils can be partitioned to minimize eddy current losses. This invention can easily be implemented in a planar technology. Simulations of several tank circuits indicate that the power dissipation can be reduced 3 to 4 times when compared to conventional techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to the co-filed U.S. applicationsentitled “FABRICATION OF INDUCTORS IN TRANSFORMER BASED TANK CIRCUITRY”,“FLUX LINKED LC TANK CIRCUITS FORMING DISTRIBUTED CLOCK NETWORKS”, and“FREQUENCY ADJUSTMENT TECHNIQUES IN COUPLED LC TANK CIRCUITS” filed onJul. 19, 2005, which are invented by the same inventor as the presentapplication and incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Electronic consumer products are pushing both the bounds of portabilityand computation complexity, in certain cases, simultaneously. Todaymobility implies that the product has attributes such as the capabilityof being wireless. In addition, since video is playing a larger role inour lives every day, the need for low power computation techniques andhigh performance for video applications for both mobile and desktopsystems is required.

An oscillator block provides the ability to regulate the flow ofcomputation data within a VLSI (Very Large Scale Integration). Forinstance, the on chip clock frequency of a high-end microprocessor isexpected to reach 10 GHz before the end of this decade. In addition, thepower dissipation for the microprocessor is expected to be about 200 W,where the clock network will consume almost half of this power or 100 W.Thus, for this microprocessor, the higher frequencies and larger powerdissipation values indicate a need to have clock circuits that caneasily generate a 10 GHz signal and should be able to reduce the powerdissipation of the clock network. The clock network of these VLSI chipstypically contains large values of capacitance that need to be driven.

Handheld units are driving the desire for the ubiquitous need forwireless. Due to the limited energy storage ability of batteries, energyconservation is paramount for longer play and talk times. These unitscontain a mixture of analog and digital components. Analog circuits areused in the radio frequency (RF) sections of the wireless blocks thattypically contain some form of a clock oscillator. The digital circuitswill require a lower power technique of distributing the clock signalwithin the chip. By minimizing the power dissipation of the clockcircuits and networks of the wireless units, the time between chargingthe batteries of the portable units can be extended.

Some of the basic circuit blocks to help achieve the ability formobility, low power, and high computation require the necessity of aclock oscillator block. Tank circuits have been used to generateoscillatory clock signals. These circuits use LC (inductor-capacitor)elements to form the tank circuit.

For example, U.S. Pat. No. 5,396,195 issued Mar. 7, 1995 to Gabaradepicts a basic LC tank circuit in an MOS technology. Several examplesare given where a cross-coupled MOS circuit drives the tank circuit. Theoscillations generated by the MOS LC tank circuit fabricated in a 0.9 μmCMOS technology operated with a supply voltage of 3.3V. The powerdissipation was reduced by a factor of a 10× when a capacitive load wasdriven using an LC tank circuit as compared to being driven usingconventional digital techniques. This circuit has been used in amultitude of applications ranging from wireless to on-chip clockgeneration modules. Many of the inductors used in this type of tankcircuit have the form of the horizontal planar inductor as illustratedin FIG. 1 a and FIG. 1 b. These type of inductors typically require alarge amount of area to form the inductor.

The calculations of the values of these type of inductors is provided ina published paper, “Simple Accurate Expressions for Planar SpiralInductances”, IEEE J. Solid-State Circuits, Vol. 34, No. 10, Oct. 1999,by Mohan et al., hereafter referred to as the “Mohan” reference.

In addition, the Q or quality factor of these inductors that arefabricated in CMOS are typically low. The quality factor or Q is aprimary parameter in the evaluation of tank circuits. $\begin{matrix}{Q = {2\pi\frac{{Maximum}\quad{energy}\quad{stored}\quad{in}\quad{tank}\quad{circuit}}{{Energy}\quad{dissipated}\quad{per}\quad{cycle}}}} & (1)\end{matrix}$The Q indicates the amount of energy dissipated by the tank circuit tomaintain oscillations. The tank circuit is more energy efficient as thevalue of the Q term increases which indicates that the energy dissipatedin the tank circuit decreases. One way to decrease the dissipation is toreduce the parasitic resistance of the inductor.

Another method to increase the Q for designs above 1 GHz is to reducethe induced eddy current in the conductor of the inductor. As pointedout by Niknejad and Meyer, IEEE Trans. Microwave Theory Tech., Vol. 49,No. 1, Jan. 2001, the eddy current loss within the metallic region ofthe planar inductors is a dominant loss above 1 GHz.

U.S. Pat. No. 6,759,937 issued Jul. 6, 2004 to Kyriazidou suggests abalanced vertical multi-layer planar inductor to reduce the area andimprove the symmetry of the inductor. A vertical planar inductor is verysimilar to a helix. This helix uses a square coil instead of a circularone. A square helix structure is illustrated in FIG. 1 c. This structureoffered the benefit of using less real estate and higher Q for a givenvalue of inductance. Kyriazidou achieves this in part by decreasing theresistance of the coil. Their approach is to shunt sections of a lowermetal layer winding to sections of a higher metal layer winding by usingmultiple vias.

U.S. Pat. No. 5,831,331 issued Nov. 3, 1998 to Lee proposed a helixstructure to form a vertical multi-layer planar inductor that usesshielding to increase the inductance. A shield formed in the substratestops the flow of eddy currents in the substrate. This structure alsooffered the benefit of using less area for a given value of inductance.In addition, Lee desired to decrease the resistance of a coil in a lowermetal layer by electrically connecting the lower layer coil to an upperlayer coil formed in a higher metal layer. There is a drawback to thisreduction of the resistance. As described by Lee, a single via is usedto create this electrical connection. Because only one end of the upperlayer coil is DC connected to the lower coil (by this single via), thedesire to reduce the resistance of the lower coil in not effective sincecurrent entering the higher metal level would not have a return pathback to the lower coil. This is in stark contrast to the approach ofKyriazidou since Kyriazidou does provide multiple current return pathsfrom sections of the upper metal layer to sections of the lower layerand achieves the goal of reducing the resistance of the coil. Thus,Lee's approach to reducing the resistance of a coil does not achieve itsgoal.

U.S. Pat. No. 6,480,086 issued Nov. 12, 2002 to Kluge et. al., describesa vertical multi-layer planar inductor to increase the inductance for agiven area usage. Kluge uses a helix to create the inductance. Inaddition, a transformer is described where the second coil is closelyspaced to the first coil to achieve a magnetic coupling between the twocoils. Kluge indicates the use of multiple vias to reduce the seriesresistance. However, this resistance reduction is directed to the viaconnection itself.

Because real estate is expensive, reducing the area used to from theinductors would be beneficial. In addition, it is desirable to addresspower dissipation reduction issues in the design of inductors. The firstis to decrease the parasitic resistance of a coil so that losses areminimized. Next, it is desirable to decrease the eddy current losswithin the metallic inductor. Doing so offers an increase in the Q ofthe tank circuit and provides the added benefit of reducing the powerdissipation of the tank circuit. This application will address these andother issues necessary to help achieve these goals.

BRIEF SUMMARY OF THE INVENTION

Inductors are used in a variety of circuits. In the manufacture of theinductor, there is a parasitic resistive element contained within theinductor. This parasitic element causes losses in the circuit. The goalis to reduce the value of this parasitic resistance as much as possiblein order to minimize the energy loss as indicated by the previousreferences. This procedure effectively lowers the sheet resistance sincethe overall metal sheet resistance decreases. However, it would bedesirable to decrease the parasitic resistance, decrease the flow ofeddy current within the metallic conductor of the inductor, and adjustthe mutual coupling between two parallel-connected inductors therebycontrolling the value of the overall effective inductance.

The basic invention is to place additional inductors in parallel acrossthe two leads of an existing inductor that forms an LC tank circuit. Itis important to note the parallel connection implies a true parallelconnection; the added inductor has two leads (or the two access pointsof the inductor) and these two leads are placed in electrical contact(in parallel) with the two leads of the existing inductor that forms theinductance in the LC tank circuit. Connecting these two inductorstogether leads to a reduction of the overall parasitic resistance. Thisreduction in resistance occurs since the parasitic resistors are allconnected in parallel. It is well known that by paralleling resistors,their net resistance decreases. However, besides reducing theresistance, the inductance also decreases according to the parallel ruleapplied to inductors. Thus, both the resistance and inductance arereduced using the parallel combination technique. In other words,connecting two identical inductors in parallel creates a singleeffective inductor that has half the parasitic resistance and half ofthe initial inductance value of either inductor. In some cases, this maybe acceptable for certain applications.

Another aspect of this invention is to reduce the parasitic resistanceof a parallel combination of inductors, yet prevent the full effect ofthe parallel reduction rule to reduce the overall inductance value. Thekey aspect of this invention is to utilize the magnetic coupling betweenthe two inductors to compensate for this inductance reduction. Thestructure to obtain this behavior is known as the transformer. In orderfor this idea to function, the transformer is connected in a particularconfiguration that allows the reduction of the parasitic resistance butattempts to maintain the value of the inductance at its original value.In its simplest form, the transformer consists of two identicalinductors that have a mutual coupling coefficient k. This value can beadjusted within the range of 0 to that approaching 1. Lenz's law isutilized to increase the overall inductance by using the magneticcoupling between the two coils to effectively increase the inductance ofcoils that are mutually coupled together. Thus, when two identicalinductors in the transformer are connected in parallel and the mutualcoupling coefficient k is close to 1, the parasitic resistance decreasesin half, but the final inductance value is nearly equal to the initialinductance value of either inductor. A further benefit of this techniquein a planar technology is that an inductor can be segmented intoparallel strips along its length according to this invention and therebyreducing the eddy current loss within the inductor. By maintaining thecoefficient k large between these parallel strips, the initial value ofthe inductor can be maintained while simultaneously achieving a decreasein eddy current loss.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 a-b illustrates two planar inductor structures.

FIG 1 c presents a view of a planar inductor that has a helix structure.

FIG. 1 d depicts an equivalent circuit for the planar inductor shown inFIG. 1 a-b.

FIG. 1 e provides dimensions and parameters for several inductors.

FIG. 2 through 3 shows an RLC circuit, a physical representation and atable giving capacitance values for several frequencies.

FIG. 4 a is an RLC circuit using two non-coupling inductors inaccordance with the present invention.

FIG. 4 b-c illustrates a physical representation and a table givingcapacitance values for several frequencies.

FIG. 5 a depicts an RLC circuit using two parallel coupling inductors inaccordance with the present invention.

FIG. 5 b-c depicts a physical representation of the circuit and a tablegiving capacitance values for several frequencies.

FIG. 6 a depicts an RLC circuit using two parallel coupling inductors inaccordance with the present invention.

FIG. 6 b-c depicts a physical representation of the circuit and a tablegiving capacitance values for several frequencies.

FIG. 7 a is an RLC circuit using two anti-parallel coupling inductors inaccordance with the present invention.

FIG. 7 b-c gives a physical representation of the circuit and a tablegiving capacitance values for several frequencies.

FIG. 8 a is an RLC circuit using two parallel coupling inductors inaccordance with the present invention.

FIG. 8 b-c provides a physical representation of the circuit and a tablegiving capacitance values for several frequencies.

FIG. 9 provides the mesh current analysis circuit for two inductors inparallel.

FIG. 10 shows an equivalent inductance L_(equ) in a parallel LC tankcircuit.

FIG. 11 shows the equivalent inductance in a Colpitts circuit.

FIG. 12 depicts the graph of the frequency ratio as a function of k thatthe inventive TC tank circuit can oscillate compared to a conventionalLC tank circuit.

FIG. 13 depicts the graph of the capacitance ratio as a function of kthat the inventive TC tank circuit can oscillate compared to aconventional LC tank circuit.

FIG. 14 a-c depicts the circuits for the mesh current analysis ofseveral parallel inductors each inductor having a series resistor.

FIG. 15 a-b provides circuit equivalent models for parallel-connectedinductors with parasitic resistances that are also magnetically coupled.

FIG. 15 c provides a table of estimated parameters of severaltransformer-based inductors following the embodiment of this invention.

FIG. 16 shows an example of the inventive technique applied to atransformer-capacitor circuit configured as a Colpitts oscillator andconnected to a regenerative circuit.

FIG. 17 shows a Hartley oscillator illustrating another inventive aspectconnected to a regenerative circuit.

FIG. 18 a-f shows several examples of a regenerative circuit.

FIG. 19 a-c shows circuit schematics where a transformer, capacitors andregenerative circuit are combined together in accordance with thepresent invention.

FIG. 20 a depicts a physical description of the inventive aspect of thetransformer structure in a planar technology connected to a regenerativecircuit.

FIG. 20 b shows the circuit schematic of the physical transformerstructure given in FIG. 20 a along with the regenerative circuit.

FIG. 21 a illustrated the inventive physical structure of a multi-coiledtransformer in a planar technology in accordance with the presentinvention.

FIG. 21 b illustrated the circuit schematic of the multi-coiledtransformer in a planar technology in accordance with the presentinvention.

FIG. 22 illustrates the electrical connection between the terminals ofthe transformer with the use of multiple vias in accordance with thepresent invention.

FIG. 23 a provides the circuit description of a conventional LC tankcircuit.

FIG. 23 b-d illustrates the inventive circuit configuration of a two,three and four-coil transformer based tank circuit.

FIG. 23 e provides the simulation results of the circuits presented inFIG. 23 a-d.

FIG. 23 f depicts the simulation conditions of the simulation resultsgiven in FIG. 23 e.

FIG. 24 presents the simulated 5 GHz waveforms of the inventivefour-coil transformer circuit.

FIG. 25 a illustrates the inventive description of a two-metal layertransformer connected as an inductor with parallel coupling.

FIG. 25 b illustrates the circuit schematic of FIG. 25 a.

FIG. 26 illustrates the inventive physical structure of a multi-coiledtransformer using a only two metal layers.

FIG. 27 a depicts an inventive cross-under for the inductor of FIG. 26.

FIG. 27 b provides a second form of an inventive cross-under for theinductor of FIG. 26.

FIG. 28 a illustrates a conventional LC tank circuit.

FIG. 28 b depicts the inventive circuit in a three-coil configurationwhere the inductor of FIG. 26 is used.

FIG. 28 c provides the simulation results of the two circuits of FIG. 28a-b.

FIG. 28 d shows the simulation conditions applied to the circuits ofFIG. 28 a-b.

FIG. 29 a depicts a conventional single turn planar inductor thatoccupies the same area as the three-coil inductor depicted in FIG. 26.

FIG. 29 b provides a magnified view of the planar inductor in FIG. 29 aassuming the inductor is formed using parallel coils is in accordancewith the present invention.

FIG. 29 c illustrates a magnified view of the inset shown in FIG. 29 bindicating the capacitances between coils in accordance with the presentinvention.

FIG. 30 a depicts the physical representation of a parallel combinationof two transformers in a planar technology in accordance with thepresent invention.

FIG. 30 b presents the equivalent circuit schematic of the structureshown in FIG. 30 a in accordance with the present invention.

FIG. 31 presents a cross-under connecting two conductors with a reducededdy-current loss in accordance with the present invention.

FIG. 32 illustrates the connection of two coils each on a separate diethat are connected together using solder bumps in a MCM technology inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The LC (inductor-capacitor) tank circuit has been a fundamental buildingblock in many electrical system designs. This circuit is used in thewireless, digital, and mixed-signal designs. The basic building elementsof the LC tank circuit consist of an inductor and capacitor.

The invention is based on the discovery that a transformer can be usedto decrease the effective resistance of an equivalent inductance that isapplied to a capacitive load while maintaining a higher inductancevalue. The coupling coefficient of the transformer can be utilized toincrease the effective inductance presented to the capacitor yetsignificantly reduce the resistance of the equivalent inductance of thetransformer. The ability to reduce the resistance will offer animprovement in the Q or quality factor of the inductor.

In addition, this technique offers a degree of freedom in the design oftank circuits, which did not exist previously. For example, tradeoffsbetween single and multi-coil tank circuits can be compared. The numberof coils in a multi-coil transformer may be optimized for a particularuse. The power dissipation of several tank circuits using an equivalentinductance based on a single or multi-coil transformer can be compared.Finally, the coupling of the transformer can be utilized to injectionlock all the tank circuits formed on the chip.

Several basic examples of an LC tank circuit in different configurationswill be described and this analysis will be used to set a referencepoint so that a better comprehension of the invention can be made. Sincethe inventive tank circuit uses a transformer instead of an inductor,these circuits are called TC (transformer-capacitor) tank circuits.

Several assumptions are initially made to simplify the analysis of theinventive entity. This helps identify the key aspects of the inventionwithout losing insight. It would be very informative to see whatcapacitive loads allow the inventive circuit to operate under threedifferent frequencies: 1 GHz, 5 GHz and 10 GHz. To help achieve thisanalysis, the following assumptions will be made.

The first one will be to assume that the resistive component of the tankcircuits will not significantly affect the frequency of operation of theTC tank circuits. Thus, instead of providing an actual resistance value,a relative value of resistance will be given and this value will bescaled appropriately for each different circuit analyzed. Later, theimpact of incorporating a realistic value of resistance into the circuitwill be described. In particular, the simulation results will includethe resistive losses of the tank circuit. This realistic value willdissipate the energy in the tank circuit thereby requiring an additionalcircuit which has the ability to regenerate the energy loss in theresistive losses of the tank circuit. Various versions of thisregenerative circuit will be described.

The second one will assume that the value of the inductor L (if thecircuit only contains one inductor) or L_(equ) (determining anequivalent inductance if several inductors are used in the circuit) willbe targeted to remain constant. Initially, this inductance value will betemporally be set to 0.8 nH unless otherwise specified.

The third one will assume that the self-inductances of the coils in thetransformer are equal.

The last one will assume that the capacitive load elements in a balancedtank circuit are equal. To be more specific, if a tank circuit generatesa clock and a clock bar signal, the capacitive load attached to both ofthese nodes are identical.

It is important to understand that setting these assumptions does notlimit the range or scope of the inventive idea. The above assumptionshelp as an aid to easily identify the keys aspects of the invention.Before a TC tank circuit is utilized in an actual operating system, eachof the above assumptions will need to be re-evaluated according to thespecifications of the design parameters. Those skilled in the art willrecognize that the above assumptions do not limit the scope of theinvention.

FIG. 1 a illustrates a square inductor 1-1 with one turn. Note that thisinductor has at least two leads, 1-4 and 1-5, or ways of physicallyconnecting the inductor to a circuit. The width of the metallic trace isshown as W. A two-turn inductor 1-2 is depicted in FIG 1 b. Themeasurements of the outside diameter is shown as d_(out), while theinside diameter is listed as d_(in). In addition, the distance betweentraces is identified as S. These types of inductors can occur in an IC(Integrated Circuit), a VLSI chip, an RF (Radio Frequency) chip, a PWB(Printed Wire Board), a MEMS (Micro-Electro-Mechanical-System) die or aMCM (Multi-Chip Module). The equivalent circuit representation 1-3 ofthe coils 1-1 and 1-2 are provided in FIG. 1 d. A simplified version ofthis equivalent circuit of an inductor will be utilized in many of thecircuits analyzed in this paper to help provided the essential idea ofthis invention.

FIG. 1 c depicts a helix structure used to form an inductor 1-3. Thisinductor has two leads 1-6 and 1-7. A single turn coil 1-12 is formed ina lower metal layer, then a via 1-8 is connect this coil to the singleturn coil 1-11 in an upper metal layer. The via 1-9 connects the middlecoil to the top coil 1-10 formed in an upper metal layer

The inductors 1-1, 1-2 and 1-3 are typical for the type of inductorsfound in a planar technology layout. These inductors are also calledcoils where coil can indicate that the conductor forming the inductorhas a configuration that spans a portion of 360 degrees.

In all of these planar inductors presented, several aspects were notshown. The substrate of the integrated circuit upon which these planarinductors are fabricated is not shown. In addition, the oxide ordielectric layer surrounding the metal layers is not illustrated. Thisprovides an easier description of the structure of the inductor. Theintegrated circuit can typically have a plurality of metallization anddielectric layers. In addition, only a square inductor has been shown,however, those skilled in the art will realize that the inductor can beformed in a circular, oval, hexagonal shape or other shape, and still bewithin the scope of the invention.

Finally, the table listed in FIG. 1 e displays the parameters of severalinductors that were designed using the “Mohan” reference. The firstcolumn lists the values of the self-inductances L as 0.8, 1.6, 0.85,0.857 and 0.865 nH. The inductors are designed with 2, 4, 2.06, 2.07 and2.08 turns as indicated in the second column. The need for the atypicalvalues of 2.06, 2.07 and 2.08 turns will become apparent later. They allhave the same outside dimension as indicated by D_(out). The remainingdimensions of d_(in), W, and S are also indicated. The number of squaresforming the conductive trace of each inductor is indicated in theseventh column. The last column provides the parasitic resistor ratiowhere the parasitic resistance of the inductor with a self-inductance of0.8 nH is used as the reference. All of these inductors were designed tooccupy the same area where each inductor occupies an area of 200 by 200μm. These inductors will be used in several different tank circuits tohelp identify the invention.

The number of squares can be used with the sheet resistance value todetermine an approximate resistance. The skin effect typically increasesthe resistance of the inductors proportional to the square root offrequency; however, the skin resistance effect will not be addressed inthis discussion so that the concepts of the invention can be more easilyvisualized. For instance, the skin-depth in copper is about 0.66 μm at10 GHz. Because of this effect, the current is carried near the surfacecausing the resistance to increase as mentioned earlier.

The schematic of a simple series RLC (resistor-inductor-capacitor)circuit 2-1 is illustrated in FIG. 2 a, while a physical model 2-2 ofthe same circuit is provided in FIG. 2 b. The actual physical model maycontain the planar inductors described earlier or consist of physicaldiscrete parts. The physical models presented in the next severalfigures are of a simplified representation and are useful to understandthe structure of the tank circuits with regard to current flows andphysical placement of the basic RLC components. The capacitor C isformed using a lower conducting plate 2-3 and upper conducting plate 2-4separated by the distance d and can have a dielectric in between theplates. A metallic wire 2-5 connects the upper plate to the lower plate(although the latter physical connection is blocked from view). Notethat the capacitor has two leads as indicated at the locations where thewire connects to the plates. The wire 2-5 represents the inductor Lwhich has a self inductance of 0.8 nH. The entire circuit containsresistance. The wire forming the inductance has a resistance value andis sometimes called a lossy inductor. The capacitor plates add anotherresistance loss as well. In addition, the contacts of the wire to thecapacitor plates add more resistance.

On a first order, most of the resistance is typically contained in theinductor L and the thrust of this description will be to reduce thiscomponent of resistance. This assumption will be applied to many of thefigures in this specification and this resistance will be calledR_(equ). However, those skilled in the art will appreciate that a moreaccurate representation of the total resistance in the tank circuit willinclude the combined resistance values of all of these circuit elements.

Assume the top plate is charged to +V volts, a current flow of I isprovided in the wire 2-5. This circuit will oscillate at differentfrequencies depending on the values of the inductor L and capacitor C.FIG. 2 c provides an approximate frequency, since the R_(equ) was notincluded, of operation according to the following formula where only thevalues of C and L are used: $\begin{matrix}{f = \frac{1}{2\pi\sqrt{LC}}} & (2)\end{matrix}$

Note that the fifth column indicates the ratio of R_(equ)/R_(ref).R_(ref) was selected to be equivalent to the parasitic resistance of the0.8 nH inductor designed in FIG 1 e. Note that the value of R_(ref) isset equal to R and in the case of the planar inductor corresponds to theresistance of 82□'s of metal.

FIG. 3 a shows the schematic of a Colpitts oscillator 3-1 consisting ofone inductor L with a value of 0.8 nH, one resistor and two capacitorsC₁ and C₂. A physical representation of the Colpitts oscillator 3-2 isillustrated in FIG. 3 b. Each capacitor is formed using a set ofparallel plates. The inductance L is formed using a wire 3-3. Theinitial voltages of −V and +V volts are applied to the top plate of eachcapacitor as illustrated. Values for the circuit elements are given inFIG. 3 c. The values of C₁ and C₂ are summed together to provide thetotal value of capacitance C_(tot) across which an oscillatory signal isgenerated. The values of C_(tot) in FIG. 3 c indicate that the Colpittsoscillator can oscillate 4 times more capacitance when compared to thesimple series RLC tank circuit 2-1 given in FIG. 2, yet both circuitsstill generate the same frequency. This occurs because the twocapacitors C₁ and C₂ are connected in series (through the common ground)and have an equivalent capacitance of C_(equ) determined by combiningthese two capacitors using the series rule of combining capacitors inequation (3) where n corresponds to the number of parallel capacitors.$\begin{matrix}{{\frac{1}{C_{1}} + \ldots + \frac{1}{C_{n}}} = \frac{1}{C_{equ}}} & (3)\end{matrix}$

Note that C_(equ) given in FIG. 3 c is equal to C given in FIG. 2 andthese two circuits become equivalent after the series rule for thecombination of capacitors is applied to the circuit of FIG. 3. Thus,after the series capacitors in the Colpitts oscillator are simplified tothe equivalent capacitance, both circuits are identical. Thus, C₁ and C₂can be series combined using equation (3) and form an equivalent circuitrepresentation as indicated in FIG. 2.

This demonstrates the benefit of a Colpitts circuit where oscillatorysignals are generated in a balanced fashion. The Colpitts oscillator candrive or generate an oscillation signal across more total capacitanceC_(tot) for the same given frequency and in addition, the Colpittsoscillator also generates a clock signal on the top plate of bothcapacitors of FIG. 3 b that are 180 degrees out of phase with eachother. Since the same inductance value was used, the ratio(R_(equ)/R_(ref)) for the Colpitts oscillator is equal to one.

FIG. 4 a depicts the schematic of a dual parallel inductor oscillator4-1. It consists of two inductors L₁ and L₂, two resistors R₁ and R₂,and one capacitor C_(tot). Note that the value of the resistances R₁ andR₂ increased by 37%. These inductors of 1.6 nH have a longer metal traceas indicated in FIG. 1 d. This longer trace also caused the resistanceR_(equ) to increase. $\begin{matrix}{{\frac{1}{L_{1}} + \ldots + \frac{1}{L_{n}}} = \frac{1}{L_{equ}}} & (4)\end{matrix}$

By using equation (4) (where n corresponds to the number of parallelinductors) to combine parallel inductors, the equivalent parallelinductance L_(equ) presented to the circuit reduces to the value of 0.8nH as required by our earlier assumptions. $\begin{matrix}{{\frac{1}{R_{1}} + \ldots + \frac{1}{R_{n}}} = \frac{1}{R_{equ}}} & (5)\end{matrix}$

Equation (5) determines the equivalent resistance of n resistor inparallel. FIG. 4 has two resistors in parallel, and the equivalentresistance is found using equation (5) and is indicated in FIG. 4 c.Note that the equivalent resistance is only 0.69R. This is an unobviousadvantage of combining lossy inductors together. That is—combiningparallel lossy inductors reduces the resistance of the overallinductance network yet still provides the value of the desiredinductance value. To our knowledge, this simple technique of reducingthe resistance by utilizing parallel combination techniques to inductorshas not been incorporated in the design of LC tank circuits. Shortly, anenhancement to this inventive technique will be described.

A physical representation of the dual parallel inductor oscillator 4-2is illustrated in FIG. 4 b and specifies the placement of the twoinductors L₁ 4-4 and L₂ 4-3 in conjunction with the parallel platecapacitor C_(tot). This circuit has a ratio (R_(equ)/R_(ref)) of 0.69,which indicates that the dual parallel inductor oscillator can decreasethe equivalent resistance R_(equ) of the circuit.

According to M. E. Van Valkenburg, Network Analysis, Third Edition,1974, Prentice-Hall, Inc., Englewoods Cliffs, N.J., page 38: “When themagnetic field produced by a changing current in one oil induces avoltage in other coils, the coils are said to be coupled, and thewindings constitute a transformer.”

There are two basis mechanisms to specify the amount of coupling anddirection of the coupling in a transformer. The amount of coupling knownas the coupling coefficient, k, determines the level of mutualinductance interaction of the transformer. The value of k can range from0 (no coupling) to 1 (100% coupling). The k value indicates how muchflux from the first coil is linked to the second coil. In addition tothe k factor, the coils of the transformer are marked to indicate thedirection of this linking or coupling. That is as current enters thefirst node of the first coil, a voltage is generated on one of the nodesof the second coil. These two nodes are then marked. Depending on thevalue of k and the positioning of the two dots, the mutual inductance ofthe transformer can be adjusted significantly. The term coil andinductor are used interchangeably. Inductors have a self-inductancewhile two inductors that are magnetically (mutually) coupled form atransformer. These inductors that are mutually coupled are referred toas coils of the transformer.

For a transformer with two coils (a transformer can have more than twocoils), the two dots can be orientated in four different ways. FIG. 5 ashows the dots configured as Top-Top (T-T). The other threepossibilities are T-B, B-T and B-B, where B stands for Bottom. Theconfiguration of T-T has a similar behavior to the B-B configuration andwill be called parallel coupling, while B-T and T-B configurations havea similar behavior and will be called anti-parallel coupling. Thus, onlythe T-T (FIG. 5) and T-B (FIG.7) will be discussed.

FIG. 5 a shows a schematic of a transformer-capacitor (TC) based tankcircuit 5-1 where two parallel inductors L₁ and L₂ are connected inparallel to the capacitor C_(tot) The two inductors also have has aseries resistor R₁ and R₂, respectively. This tank circuit has a T-Tcoupling configuration. A physical depiction of this circuit is providedin FIG. 5 b. Note that the inductors L₁ and L₂ are formed from two wireswhich each connect the top plate of the capacitor to the lower plate ofthe capacitor, furthermore, these two wires are closely positioned toeach other. Since the current flow in both wires is in the samedirection, the transformer behaves as a T-T coupling configuration. InFIG. 5 a, the current flow in one coil of a T-T configuration wouldpresent more inductance to the second coil according to Lenz's law. Ascurrent flows in the first coil, it will retard current flow in thesecond coil. Likewise, the current flow in the second coil will retardthe current flow in the first coil. Thus, a T-T configuration will tendto increase the equivalent inductance in the circuit. In addition, thecoupling coefficient k indicated the strength of this interaction. Thecloser the wires are together, the greater the coupling coefficient. InFIG. 5 b, the wires forming the inductances L₁ and L₂ are placedadjacent to one another, thus the coupling coefficient k would be large;assume it is 0.9. Because of the T-T configuration and k being large,the mutual inductance the transformer presented to the capacitor C_(tot)is large; the equivalent inductance L_(equ) presented to the capacitorC_(tot) is almost equal to the self-inductance of either coil or 1.58nH. Again, because the larger inductance value was used, thecorresponding resistance of each inductor has a value that is 37%greater. However, the parallel combination of these two resistor givesan equivalent resistance of R_(equ)=0.69R and is an improvement overusing a single inductor as described earlier.

Because the equivalent inductance L_(equ) of the transformer in FIG. 5 aworks out to be 1.58 nH, the desired goal of achieving 0.8 nH wasovershot due to the mutual inductance of the transformer causing theinductive value to increase. This implies that the initial value of theeach self-inductance in the transformer can be adjusted to compensatefor the effect of the mutual inductance. However, this time the couplingcoefficient k is used to determine the starting value of each inductorL₁ and L₂ to insure that the final parallel combination of the L_(equ)solves to 0.8 nH. If k is assumed to be 0.9, the inductor values areestimated to be 0.85 nH each. Doing so and using k =0.9, provides afinal quivalent inductance L_(equ) of 0.8 nH.

This is illustrated in FIG. 6, which duplicates the circuit of FIG. 5with the exception that the value of the self-inductance of theinductors L₁ and L₂ are set to 0.85 nH. If these two parallel inductorsare simplified to their equivalent inductance using the couplingcoefficient k and the T-T coupling configuration, the equivalentinductance is 0.8 nH as indicated in FIG. 6 c.This is spectacularbecause the resistance of the two 0.85 nH inductors comprising thetransformer is only 1.017R. After determining the equivalent resistanceR_(equ) of these two parallel-connected resistors R₁ and R₂, the valueof R_(equ) is found to be 0.51R. The ratio R_(equ)/R_(ref) is 0.51.Thus, the transformer tank circuit whit a T-T coupling coefficient and aK value of 0.9 reduces the resistance down to 50% of the Colpittsoscillator given in FIG. 3. This is the key aspect of the invention thatmaintains a desire value of inductance yet minimizes the finalresistance. Thus, this circuit would have a higher Q or quality factorsince the resistive loss would decrease.

This idea can be extended to include a transformer having more than twoinductors where the equivalent resistance will be the parallelcombination of three or more parallel resistors causing the resistanceto become further reduced. This is an aspect that all tank circuits seekto achieve since the a higher Q provides many benefits includinggenerating less phase noise, operating with better frequency selection,and having less power loss to name a few characteristics.

In FIG. 7 a, the schematic of a circuit 7-1 using the T-B anti-parallelcoupling configuration is illustrated. Here the current flow in one coilwould enhance the flow of current in the second coil; likewise, thecurrent flow in the second coil would also enhance the current flow inthe first coil. Thus, the T-B configuration reduces the equivalentinductance being presented to the capacitor C_(tot) depending on thevalue of the coupling coefficient k.

FIG. 7 b illustrates the physical structure 7-2 of the circuit in FIG. 7a. The upper conducting plate 7-3 and the lower conducting plate 7-4which are separated by the distance d forms the capacitor C_(tot). Ametalic wire 7-5 makes a connection 7-9 to the upper plate 7-3 and loopsaround both plates to make a connection 710 to the lower plate 7-4. Inaddition, a second wire 7-6 connects to the top plate 7-3 and loopsdownward through the hole 7-8 in the bottom plate 7-4 and then loopsaround both plates passing through the hole 7-7 in the upper plate 7-3that then makes a connection 7-10 to the lower plate 7-4. The wire 7-5is the coil forming the inductance L₁, while the wire 7-6 is the coilforming the inductance L₂. Because these two coils are placed closetogether, the coupling coefficient k should be large. The lower plate7-4 is connected to ground.

FIG. 7 c provides capacitance value and the frequency of operation ofthe circuit. Note that the equivalent inductance that the transformerpresents to the capacitor C_(tot) is very low—only 0.04 nH when k isassumed to be 0.9. This illustrates the case where the equivalentinductance L_(equ) can be significantly reduced. This type of circuitcan be used to reduce the inductive voltage drop of a varying current,since the voltage drop will be proportional to the inductance.

The TC tank circuit can be arranged to have a Colpitts configuration asindicated by the circuit schematic 8-1 given in FIG. 8 a. Thistransformer uses a T-T coupling configuration. The correspondingphysical structure 8-2 is indicated in FIG. 8 b. The two capacitors C₁and C₂ are formed using parallel plate conductors. The top plate of thefirst capacitor is connected to the top plate of the second capacitorusing two wires that are also the elements of the coils of thetransformer. As indicated in FIG. 8 c, the L_(equ) is 0.8 nH. Also, theequivalent resistance R_(equ) is approximately 50% of the initialcircuit which did not incorporate the mutual inductance effect. Finally,note that the circuit in FIG. 8 can drive into oscillation 4 times morecapacitance than the circuit illustrates in FIG. 6.

The coupling coefficient can be utilized in the design of tank circuitsto affect the value of inductance that is presented to the capacitivecomponent of a tank circuit. This ability can be used for a multitude ofuses in the analog and digital field; such as, wireless applications,clocking networks, driving mixers, adiabatic logic, to name a few.

FIG. 9 is used to determine the effective inductance of the parallelcombination of L₁ and L₂ that are magnetically coupled. Note that thetwo coils are in a parallel coupling configuration. The mutualinductance is given as:M=k√{square root over (L₁L₂)}  (6)

Using mesh equations, the equivalent inductance (L_(equ)) of the coupledcoils 9-1 of FIG. 9 is: $\begin{matrix}\begin{matrix}{Z_{input} = \frac{\Delta_{z}}{\Delta_{11}}} \\{= {{j\omega}\quad L_{equ}}} \\{= {{j\omega}\frac{\begin{matrix}L_{1} & {{- L_{1}} + M} \\{{- L_{1}} + M} & {L_{1} + L_{2} - {2M}}\end{matrix}}{L_{1} + L_{2} - {2M}}}} \\{= {{{j\omega}\quad L_{1}} - {{j\omega}\frac{\left( {{- L_{1}} + M} \right)^{2}}{L_{1} + L_{2} - {2M}}}}}\end{matrix} & (7)\end{matrix}$

Assume that L₁=L₂=L and using equation (6) gives; $\begin{matrix}{L_{equ} = {\frac{L}{2}\left( {1 + k} \right)}} & (8)\end{matrix}$If the transformer had the two coils in an anti-parallel couplingconfiguration, the + sign in equation (8) would be changed to a − sign.

Placing a capacitor across the equivalent inductance L_(equ) forms thetank circuit 10-1 given in FIG. 10. One end of the capacitor is groundedand a single output is generated at 10-2.

A Colpitts oscillator 11-1 is illustrated in FIG. 11. Two capacitors C₁,and C₂ are placed at each end of the equivalent inductance L_(equ)providing two outputs 11-2 and 11-3 which generate oscillations that are180 degrees out of phase with each other. The capacitors C₁ and C₂ inFIG. 11 are both twice the value of the single capacitor C in FIG. 10and both circuits will operate at the same frequency.

The ratio of the frequency of a TC tank circuit f_(TC) compared to thefrequency of a LC tank circuit f_(LC) is illustrated in FIG. 12. TheseTC tank circuits contain two self-inductances. Equation 2 and equation 8are used to determine the ratio of the frequencies of these twodifferent types of tank circuits. The final simplified relationship isgiven as follows: $\begin{matrix}{\frac{f_{TC}}{f_{LC}} = \frac{1}{\sqrt{1 \pm k}}} & (9)\end{matrix}$

The relationship given in equation (9) is plotted in FIG. 12. The kvalue has either a + or − term to account whether the circuit is aparallel or anti-parallel configuration, respectively. Note when k=0,the ratio equals one as expected. The anti-parallel couplingconfiguration occurs when the denominator is (1−k). This corresponds tothe top curve 12-1. The frequency of the TC tank circuit increases andapproaches infinity as k−>1.

The lower curve 12-2 corresponds to the case when the denominator is(1+k), this is the parallel coupling configuration and indicates thatthe effective inductance of the tank circuit increases and lowers thefrequency of operation of the TC tank circuit as compared to the LC tankcircuit.

The next relationship given in equation (10) is plotted in FIG. 13. Fora given k value, this graph compares the amount of capacitance C_(TC) ina TC tank circuit to the amount of capacitance C_(LC) in a LC tankcircuit that can be placed into oscillation as a function of thecoupling coefficient. $\begin{matrix}{\frac{C_{TC}}{C_{LC}} = \frac{1}{1 \pm k}} & (10)\end{matrix}$

The anti-parallel coupling configuration occurs when the denominator is(1−k), as the coupling coefficient k increases, the amount ofcapacitance 13-1 that the TC tank circuit can oscillate increases. Thisoccurs because the equivalent inductance of the TC tank circuitdecreases.

The lower curve 13-2 corresponds to the case when the denominator is(1+k), the amount of capacitance that the circuit can oscillatedecreases at a slow rate as the function k −>1.

The mesh equation analysis for all the schematics given in FIG. 14 wereperformed similar to the analysis determined in FIG. 9. Besides having aseries resistance, each coil has a mutual inductance term M with everyother coil in the circuit. The analysis given is for the case wherecoils of the circuit all are arranged in a parallel coupling fashion.That is, all the coils are configured in an arrangement of T-T- . . .−T. Assume that all self-inductances; L₁=L₂ . . .=L, that allresistances; R₁=R₂ . . .=R, and all mutual inductances; M₁, M₂ . . . =M.

This assumption simplifies the equation and provides an insight intofinding an approximate value for the equivalent inductance. Acceptingthe previous assumptions and conditions, the equivalent impedance Z₂ fora two-coil transformer for the circuit using the current mesh analysisdepicted in FIG. 14 a is: $\begin{matrix}{Z_{2} = {{R_{equ} + {{j\omega}\quad L_{equ}}} = {\frac{R}{2} + {{j\omega}\quad\frac{L}{2}\left( {1 + {2k}} \right)}}}} & (11)\end{matrix}$

FIG. 14 b shows a parallel connection of three mutually coupled coils.The equivalent impedance Z₃ for this three-coil transformer is:$\begin{matrix}{Z_{3}\quad = \quad{{R_{equ}\quad + \quad{{j\omega}\quad L_{equ}}}\quad = \quad{\frac{R}{3}\quad + \quad{{j\omega}\quad{\quad\frac{L}{3}}\quad\left( {1\quad + \quad{2\quad k}} \right)}}}} & (12)\end{matrix}$

FIG. 14 c shows a parallel connection of four mutually coupled coils.The equivalent impedance Z₄ for this four-coil transformer is:$\begin{matrix}{Z_{3} = {{R_{equ} + {{j\omega}\quad L_{equ}}} = {\frac{R}{4} + {{j\omega}\frac{L}{4}\left( {1 + {3k}} \right)}}}} & (13)\end{matrix}$

Note two conditions in equations 11 through 13; if k is close to 1, theequivalent inductance L_(equ) is approximate to the value of theself-inductance L of a single coil. Secondly, the resistance of themulti-coil transformer decreases proportionally to the number of coils.Thus, this type of transformer provides an inductance that remainsconstant but decreases in resistance as the number of coils areincreased as indicated by the resistive component R_(equ) of equation(11) through equation (13). This will provide a mechanism to improve theQ or quality factor of an inductor in circuits where a high Q isdesired. In addition, the power dissipation of the circuit will bedecreased.

FIG. 15 a depicts this impedance in a Colpitts oscillator 15-1configuration including the equivalent resistance R_(equ). The totalresistance in a tank circuit consists of several resistance terms.Because of this resistive loss, the oscillations generated by the tankcircuit 15-1 would eventually die out. Thus, a regenerative circuit isrequired to replace the energy lost by the dissipative process of energyflow through the resistive components. The regenerative circuit can beformed out of active devices; such as MOS transistors, CMOS transistorsor BJT transistors. The regenerative circuit provides a negativeresistance that cancels the parasitic resistance in the tank circuit.FIG. 15 b combines the equivalent resistance R_(equ) and the equivalentinductance L_(equ) into an inductor symbol 15-2 with two dots calledL_(TC). The two dots indicate whether the transformer has a parallel oranti-parallel configuration. The symbol 15-2 is a parallel-configuredtransformer.

FIG. 15 c provides the parameters of a coil and several transformersthat can be formed in a planar technology. The single coil is listed inthe first row where T=1. This coil has two turns and is the same coildescribed the first row of FIG. 1 e. The self-inductance L is 0.8 nH andthe metallic trace has 82□'s of resistance. Since this is only a singlecoil, the values of L_(equ) and R_(equ) have the same values as before,respectively. Assuming a sheet resistance of 0.08Ω/□, the resistance ofthis single coil is 6.56Ω. The resistance of R_(coil) and R_(equ) areequivalent since there is only one coil. The Q of this coil is 0.765.

The second row of FIG. 15 c gives the parameters for a transformer withtwo coils since T=2. The coupling configuration of this and theremaining coils all has a parallel coupling configuration as in thiscase indicated by the T-T. This transformer, as well as, the remainingtransformers is formed using the single coil listed in the first rowwith a slight modification. This modification is that the number ofturns has been increased to 2.06. For the transformer with T=2, the Nhas been increased to 2.06. This modification increases theself-inductance of the coil to 83.4 nH. Referring to FIG 1 e, note thatthe outer dimension of this coil remains at the value of D_(out)=200 μm,Thus, the area of this transformer is the same as that of the singlecoil. The coupling coefficient k is 9.0. Using equation (11), theL_(equ) is found to be 0.8 nH. The value of the resistance R_(equ) forthis transformer is given as 3.34Ω. This lower value for resistanceoccurs because each coil has a resistance of 6.67Ω due to the turnsration of 2.06. Since the two coils are in parallel, the equivalentresistance Requ is the parallel combination of the two resistances or3.34Ω. The Q for this transformer is 1.5.

The third and fourth rows indicate the parameters of a 3 and 4 coiltransformer. The turns ratio of the coils forming the transformer is2.07 and 2.08, respectively. In both transformers, the self-inductance Lhas been designed at 0.857 and 0.865 nH, respectively. Using equation(12) and equation (13), respectively, the equivalent inductances L_(equ)of these two transformers are found to be 0.8 nH. Similarly, theresistance for the three coil transformer (T=3) R_(equ) is 2.22Ω. In thecase of the T=4 coils transformer, the resistance R_(equ) is only 1.68Ω.This is over a 4× reduction over that of a single coil. Note that the Qhas been increased to almost 3.

This is the key aspect of this invention—paralleling multi-coils in aparallel coupling configuration allows an equivalent inductance L_(equ)of a single coil to be achieved with a reduced equivalent resistanceR_(equ) proportional to the number of coils used in the transformer.Furthermore, the area of all of these transformers can be containedwithin the original area of a single coil. The capacitor loads C1 and C2can consist of all the parasitic capacitance in the entire circuit. Inaddition, this capacitor can contain the load capacitance and theadjustable capacitor.

An example of a Colpitts oscillator 16-1 connected to a regenerativecircuit 16-2 is given in FIG. 16. This circuit has two outputs 16-3 and16-4 that connects to the tank circuit. This regenerative circuit istype regen-1 and in addition has a power and ground connection. Theoscillator signal is generated across the two capacitors C₁ and C₂. Thesignal that is developed across these two capacitors are 180 degrees outof phase with each other.

A Hartley oscillator 17-1 is shown in FIG. 17. In total, this circuitrequires four inductors. This regenerative circuit 17-2 is type regen-2and has a ground in this case, but as will be seen shortly may onlycontain a power connection. The regen-2 has two outputs 17-3 and 17-4that connect to the tank circuit. Note that both the Colpitts and theHartley use the parallel coupling configuration. Depending on the need,these oscillators can be configured as anti-parallel couplingconfiguration as well.

FIG. 18 illustrates several CMOS circuits configured in either regen-1or regen-2 type regenerative circuits. Similar circuits can be designedusing BJT transistors as well. In FIG. 18 a, a regen-1 type circuit 18-1is shown; here a p-channel 18-2 serves as a current source to the restof the circuit. The two p-channel device 18-3 and 18-4 are cross coupledto each other; that is, the drain of 18-3 is connected to the gate of18-4 and the drain of 18-4 is connected to the gate of 18-3 This forms aregenerative circuit. A second regenerative circuit consists of the twon-channels 18-5 and 18-6 configured in a similar manner. The negativeresistance of this circuit is provided to the TC tank circuit using thetwo outputs 18-7 and 18-8. The negative resistance of the circuitcompensates for the resistive loss of the transformer and allows theoscillation created in the tank circuit to continue.

A regen-1 type circuit 18-10 similar to 18-2 is given in FIG.18 b. Thep-channel current source has been removed.

An equivalent representation of the circuit 18-9 is provided in FIG. 18c. This regenerative circuit 18-10 consists of two inverters connectedhead to tail as shown. This is also the basic building block of aram-cell that is used to store memory in integrated circuits (IC).

The remaining circuits are regen-2 type. In FIG. 18 d, only twocross-coupled n-channel devices form the circuit 18-11 that is used tocompensate for the resistive loss of the TC tank circuit. The outputsfor this circuit are 18-13 and 18-14. These are the two nodes connectedto the TC tank circuit and any external load that is desired to bedriven.

The circuit shown in 18-12 of FIG. 18 e includes an n-channel currentsource 18-13. Otherwise it is similar to the circuit of 18-11.

The last circuit 18-14 illustrated in FIG. 18 f is the compliment of18-11, that is all the n-channels are replaced by p-channels and the VSSpower supplies (ground) replaced by VDD supplies and vice-versa.

FIG. 19 illustrates the TC tank circuit with a regenerative circuit tocompensate for all the resistive losses within the tank circuit. Inaddition an adjustable and capacitive load is shown connected to bothoutputs of the TC tank circuit. FIG. 19 a depicts a current controlledregenerative circuit driving a balanced capacitive load 19-1. Acapacitor 19-2 combines all of the capacitance that is typicallynon-adjustable. This may include the parasitic capacitance of wireinterconnections, the capacitance of the gate, drain, overlapcapacitance of the transistors forming the regenerative circuit, thecapacitance of the inductors 19-4 forming the transformer, and thecapacitance of the gates or circuits being driven by the TC tankcircuit.

The capacitor 19-3 is an adjustable capacitor which is used to adjustthe frequency of oscillation of the TC tank circuit. Some examplesinclude a voltage-controlled varactor that can be formed using a diodeor a MOS transistor. The MOS device can be configured as an enhancementor depletion mode device. By adjusting the control voltage to thesedevices, the capacitance presented to the tank circuit can be modified,thereby, modifying the frequency of operation of the tank circuit.Another form of adjustable capacitor would include an array of MOStransistors. The array would present capacitance to the TC tank circuitthrough switches that can be controlled by a set of control voltages. Byadjusting these voltages, one or many gates can be connected ordisconnected to the TC circuit which in turn varies the effectivecapacitance presented to the TC tank circuit. The frequency of operationof the tank circuit changes according to equation (2). The inductance ofthe tank circuit 19-4 contains the transformer which has an effectiveinductance of L_(equ).

FIG. 19 b shows a TC tank circuit 19-4 that is identical to the circuitof 19-1 except the current controlled p-channel transistor is removed.The circuit 19-4 can generate a voltage swing that is larger than theswing produce by the circuit 19-1.

Finally, FIG. 19 c presents a TC tank circuit 19-5 that replaces thetransistors in the circuit of 19-4 with a ram-cell circuit for a compactrepresentation of a TC tank circuit.

FIG. 20 a and FIG. 20 b illustrate a Colpitts oscillator 20-1. Inparticular, the physical structure of the lower 20-2 and upper 20-3inductor coils of the transformer are given in FIG. 20 a while thecorresponding circuit representation of the lower 20-2 and upper 20-3inductor coils are given in FIG. 20 b. The lower and upper coils can beformed in a planar technology. Note that the substrate, vias, and oxideor dielectric layers are not shown with the coils so that thedescription is simplified. An upper metal layer can be used to fabricatethe upper coil, while a lower metal level can be used to fabricate thelower coil. In addition, a dielectric layer would be used to separatethe two metals while vias would penetrate the dielectric and formelectrical connections to the ends of the two inductors.

Both FIG. 20 a and FIG. 20 b contain the circuit representation of theregenerative circuit (the two inverters), the adjustable capacitor 20-8and the non-adjustable capacitor 20-9 and 20-10 loading both sides ofthe regenerative circuit. Note that the adjustable capacitor is betweenthe two outputs of the tank circuit. This helps to reduce the arearequired since this single capacitor can occupy an area of ¼ that of twoseparate adjustable capacitors connected to each output.

In both FIG. 20 a and FIG. 20 b, one side of the regenerative circuit20-4 is sourcing current 20-6 into both the upper 20-3 and lower 20-2inductors. This sourcing current consists of stored charge from loadcapacitor 20-9, one plate of the adjustable capacitor 20-8 and from theoutput of the lower inverter. The other side of the regenerative circuit20-5 sinks the current 20-7 from both the upper 20-3 and lower 20-2inductors. This current is stored onto the load capacitance 20-10, thesecond plate of the adjustable capacitor 20-8, and routed by the upperinverter of the regenerative circuit.

FIG. 20 a illustrates the physical structure of the transformer. Notethat the inductor 20-3 overlays the inductor 20-2. The number of turns Nin the coil can be a variable. Such a transformer can be fabricated inintegrated circuits (IC) where a lower metal layer can be used to form20-2 while an upper metal layer can be used to form 20-3. Furthermore,this example shows the case where the coils only have one turn (N=1).

FIG. 20 b presents the circuit equivalent of the transformer. The upper20-3 coil consists of the self-inductance L₁ and resistance R₁. Thelower 20-2 coil is represented by the self-inductance L₂ and resistanceR₂. The coils have a mutual inductance denoted by M. The dots on thetransformer indicate that the coils are arranged in a parallel couplingconfiguration. Thus, the coils are arranged to have an equivalentinductance L_(equ) that attempts to match the value of theself-inductance of either coil as indicated by equation (11) where the +sign is taken. The larger the value of k, the greater will be the match.Furthermore, by observing that the value of the resistance in equation(11) is reduced to half that of a single coil or inductor, making amulti-layer transformer would have the additional benefit of reducingthe equivalent resistance. The reduction of the resistance occursbecause the coils are connected in parallel.

A physical example of a vertical multi-coil transformer 21-1 isillustrated in FIG. 21 a. Each inductor is formed in a different metallayer of the planar technology. The illustration indicates a coil with asingle turn; however, the coil may contain several turns as well. For aplanar technology, these inductor coils have a width W that may be inthe 10's of microns wide, the thickness t of the metal layer may be inthe range of a micron or so, while the displacement d between metallayers may also be in the range of a micron or so. The overalldimensions of the transformer can be a hundreds of microns by a hundredsof microns. The top most coil 21-2 may be fabricated by using the topmetal layers, the coil 21-3 below is fabricated in one of the lowermetal layers, similarly, coils 21-4 and 21-5 are fabricated in the lowermetal layers. In addition, the coils may be positioned or aligned overone another as shown. Note that the influence of the capacitance betweenthe coils 21-2 and 21-3, 21-3 and 21-4, and 21-4 and 21-5 the potentialson these coils are identical to one anther.

The ingress current 21-8 is provided into each of the four inductors asindicated by the arrows 21-6. This is the ingress point of all fourinductors. The egress current 21-7 from each inductor coil is collectedand sunk as current 21-9. This is the egress point for all fourinductors. Note that a symbolic short connects all four-ingress points,in addition, a symbolic short connects all four-egress points. Forexample, the ingress points of coils 21-2 an 21-3 are connected togetherby the short 21-10, while the ingress points of 21-3 and 21-4 areconnected by the short 21-12. Similarly, the egress points of coils21-2, 21-3 and 21-4 are connected together by the shorts 21-11 and21-13, respectively. Vias will be used to replace these shorts 21-10,21-12, 21-11, and 21-13 a planar technology as will be shown shortly.Since there are four coils connected in parallel, the equivalentresistance R_(equ), assuming each coil has the same resistance R, wouldbe R divided by the number of coils, or R_(equ)=R/4. Thus, multi-coiltransformers have the ability to reduce the resistance substantially.

This structure can also be implemented in the MEMS or MCM technology.The MEMS offers the ability to adjust the position of one of the coilswhich directly adjusts the coupling coefficient k. This feature can beused to adjust the frequency of oscillation of the tank circuit. In aMCM technology, the dimensions and spacing between coils can beincreased since another die can be solder bumped to another die. Thetransformer can be split between these two die. The bump height can bemore than a few microns.

So far the use of this transformer has been described in a tank circuit.However, those skilled in the art will recognize that this type oftransformer circuit can be utilized in other circuit structures outsidethe domain of tank circuits. For example, this structure can be usedwherever inductors are used in circuit and system designs. The use ofthis technique has a broad range in circuit applications, such as,filters, power supplies, RF circuits, mixers, etc.

FIG. 21 b provides the schematic representation of the physicalmulti-coil transformer provided in FIG. 21 a. Assume all coils have thesame inductance L. The schematic shows the transformer consisting offour inductors 21-14, 21-15, 21-16 and 21-17. These inductors representthe coils 21-2, 21-3, 21-3 and 21-4, respectively. Note from the mutualcoupling dots that this arrangement is in a T-T-T-T (parallel coupling)arrangement. Thus, the mutual magnetic coupling will tend to maximizethe amount of equivalent inductance that this circuit presents at itsterminals 21-18 and 21-19.

FIG. 22 illustrates the physical connection 22-1 showing how the ingressand egress points of the top three inductors; 21-2, 21-3 and 21-4 ofFIG. 21 are connected together. The arrow 21-6 in FIG. 22 is the ingresscurrent provided into the top coil on the left side of the inductors ofFIG. 21. The ingress point of the top two coils 21-2 and 21-3 areconnected together by collection of metallic vias 22-8 which correspondsto the short 21-10 in FIG. 21. These vias are used in the technology tointerconnect two metal layers. In addition the third coil 21-4 isconnected to the second coil 21-3 using the vias 22-9 which correspondto the short 21-12 in FIG. 21. The arrow 21-7 corresponds to the egresscurrent from the top coil on the right side of the inductors of

FIG. 21. In a similar manner, the metallic vias 22-10 and 22-11 in FIG.22 correspond to the shorts 21-11 and 21-13 in FIG. 21. Thus, thesecoils are shorted at both ends by the vias, therefore, the equivalentresistance R_(equ) of all four coils is the parallel combination of theindividual resistances of each coil.

A Spice simulation was performed to demonstrate the benefit of usingmulti-coil transformers in tank circuits. FIG. 23 f provides thesimulation conditions and are given in the table 23-5. The frequency ofoscillation of the tank circuit was performed at 5 GHz. A 0.18 μm CMOSprocess technology operating at a VDD of 1.8V was assumed. To equalizethe results of all four tank circuits, the output waveform of the tankcircuits was designed to swing between 0.1V and 1.6V. In addition, thesheet resistance of all metal layers was assumed to be 0.08Ω/□.

The circuits that were simulated are illustrated in FIG. 23 a-d. Allcircuits have a regenerative circuit consisting of two cross-coupleinverters. Note that the equivalent resistance R_(equ) of each inductorwas portioned in half and positioned on each side of the correspondinginductor as R.

The conventional LC tank circuit 23-1 in FIG. 23 a has only oneinductor. The results of this circuit provide the reference point forthe remaining circuits. In FIG. 23 b, a transformer with two coils 23-2having a parallel coupling is indicated. This is the one version of theTC tank circuit mentioned earlier. A three-coil transformer 23-3 isdepicted in FIG. 23 c. All inductors are arranged to have parallelcoupling. Finally, FIG. 23 d provides the multi inductor using fourinductors 23-4.

Some of the results of the simulation 23-6 of the four differentcircuits are given in the table of FIG. 23 e. The top row indicates thetype of circuit that was simulated. The second column is theconventional LC tank circuit, the third column gives the results for thetwo-coil TC tank, while the remaining columns show the three andfour-coil results. In order to achieve a 5 GHz operation with an outputvoltage swing varying from 0.1V to 1.6V, both the size of the invertersand the value of the capacitor had to be adjusted. The first row of 23-6provides the required widths of the p-channel and n-channel of theinverter for the circuit just above their respective column. The secondrow indicated the capacitive load that can be driven in addition to theparasitic capacitance of the inverters.

Due to the large resistive loss of the inductor, the results for theconventional LC tank circuit indicate the need for a large inverter: 200μm/100 μm, the self-capacitance of this large gate minimizes the amountof external load capacitance that can be driven. In this case, the valueof C can only be 0.5 pF. In addition, because of the large inverters,the power dissipation is 21 mW.

The results of the two-coil TC tank circuit of 23-2 in FIG. 23 b aremore promising. Note that the size of the inverter in the regenerativecircuit can be decreased by 60% or to 80/40 μm. This in turn increasesthe external load capacitance (1.6 pF) that can be driven by over 150%as compared to the conventional LC tank circuit (0.5 pF). Finally, thepower dissipation dropped by nearly half to 10.7 mW.

The three-coil TC tank circuit of 23-3 in FIG. 23 c, which used athree-coil transformer, improved the situation even further. Theinverter size (50/25 μm) dropped to 25% of the conventional LC tankcircuit. In addition, the capacitive load that can be driven increasedby a factor of 4 (to 1.9 pF) and the power dropped to approximately ⅓that of the conventional LC tank circuit (7.2 mW).

The last circuit 23-4 of the four-coil TC tank circuit in FIG. 23 dcontinued the improvement, where the inverters dropped to a ⅕ of thesize given in the conventional design. The power dissipation droppedalmost to a ¼ as compared to the conventional LC tank circuit while thecapacitive load increased by a factor of 4×.

Thus, the simulation results confirm the advantage of utilizinginductors connected in parallel to provide advantages in reducing theresistance of the equivalent inductance (given in the bottom row of FIG.23 e), increasing the quality factor Q, decreasing of the powerdissipation of the circuit and increasing the amount of externalcapacitive load that can be driven into oscillation.

FIG. 24 provides the simulation results 24-1. The two sinusoidal outputs23-7 and 23-8 correspond to the outputs of the circuit in FIG. 23 d thatis the four-coil transformer circuit. Note that the frequency ofoscillation for these waveforms is approximately 5 GHz.

FIG. 25 a illustrates another transformer structure 25-1. Thistransformer consists of two interwoven coils and offers a balanceddifferential interface. The first coil 25-2 receives current from port25-6 and routes the current through the crossover 25-5 to the upperlayer. The second coil 25-3 receives current from port 25-6 through theinterconnect 25-4 and routes the current through the crossover 25-5 tothe lower layer. The current approaches the end of the coils but thetransformer has these two ends of its outputs shorted by 25-4. Thiscurrent then exits the transformer at port 25-7.

FIG. 25 b provides the schematic of the transformer identifying the samecomponents in the circuit. The transformer has four ports. Two of theports are shorted by the connection 25-4. Current from input port 25-6enters coil L₁ and coil L₂ and exits at node 25-7. Note that thistransformer has a parallel coupling configuration; this transformer willhave an increased equivalent inductance according to the equation (11).In addition, the dots indicate this coupling. Thus, this is anotherrepresentation where the two coils of a transformer are shorted togetherto effectively form one inductor.

A horizontal multi-coil transformer 26-1 for a planar technology isdepicted in FIG. 26. In this example, at least two metal layers areused. The upper metal layer contains the coils of the three co-linearinductors 26-2, 26-3 and 26-4 that is using thick metal with a sheetresistance of 0.1Ω/□. Each coil carries a current 26-8 from the primarysource 26-9. This current passes to the other end of the transformer ascurrent 26-8 and is collected and sunk a current 26-10.

In the upper right comer, the three coils route around one another. Thisis where metal vias are used to bridge the current from the upper metallayer to a lower metal layer to cross under the obstructing coil and usevias to redirect the current back to the upper metal layer. Thisstructure is called a cross-under and redirects the current 26-8 downand under the coil fabricated in the top thick metal layer. For example,in coil 26-2, a cross-under 26-5 is used to continue the current flow26-8 under the coil 26-3 and back to the coil 26-2. The middle coil 26-3crosses under the coil using the cross-under 26-6. Finally, the lastcoil 26-4 crosses under coil 26-2 using the cross-under 26-7. Note thateach coil only has one cross-under in its entire path insuring that allthe coils have a similar characteristic; for example, the resistanceterm in each coil is equalized.

FIG. 27 a and FIG. 27 b show the structure of the cross-under; inparticular, the cross-under 26-7 of FIG. 26 is enlarged to allow abetter description. In FIG. 27 a, the top metal layer shows that thickmetal is used for the inductors 26-2 and 26-4. However, as indicated inFIG. 26 and FIG. 27, the coil 26-4 crosses under coil 26-2 using thecross-under 26-7. As indicated in FIG. 27 a, this is accomplished usingvias 27-1 and one of the lower metal layers such as 27-2. Note that themetal layer 27-2 may be thinner. Thus, the sheet resistance of the lowermetal layer may be more than that of the top layer. To help reduce theresistance of the cross-under, many vias are used to help cut down ontheir contribution to the resistance. Finally, FIG. 27 b indicates theaddition of another metal layer 27-4 and vias 27-3 to further reduce theresistance of the cross-under.

A circuit simulation was performed and the simulation conditions areillustrated in FIG. 28 d. The sheet resistance of the coil was assumedto be 0.01Ω/□, which is the value typical for thick metal. The processwas 0.18 μm operating at a voltage of 1.8V. The resistance of each viawas 22Ω. For the cross-under, the sheet resistance of the thinner metalwas assumed to be 0.08Ω/□.

FIG. 28 gives the simulation results when thick metal is used. Twocircuits were simulated and are illustrated in FIG. 28. The first wasthe conventional LC tank circuit 28-1 given in FIG. 28 a. The coil usedwas assumed to have one turn and had an inductance of 0.78 nH. This coildid not have any cross-under's and was fabricated using thick metal. Thesecond simulation was performed on the three-coil TC tank circuit givenin FIG. 28 b. The layout of the multi-coil transformer given in FIG. 26was used and thick metal was used.

The results for these two simulations are given in FIG. 28 c. There is afactor of 4× in the size of the inverters between the conventional LCand the three-coil TC tank circuit. The amount of capacitance that thethree-coil TC can drive increased 20%, while the power dissipationdropped by almost 3×. This occurred because the overall resistance ofthe single coil of the conventional LC tank circuit was reduced from1.14Ω to 0.367Ω for the three-coil transformer used in the three-coil TCcircuit.

An analysis was performed to determine the value of the equivalentinductance and parasitic resistance of a conventional single turn coilshown in FIG. 29 a that would occupy the same area as the three-coiltransformer in FIG. 26. Because of scaling the values for the resistanceof these two different structures are comparable. In addition, theinductances are comparable. The advantage of the horizontal multi-coiltransformer is that the eddy current loss can be decreased in themulti-coil transformer.

Moving forward with this analysis, the conventional single turn coil ofFIG. 29 a is transformed into a multi-coil transformer to help show thereduction in the eddy current loss. The magnified region 29-7 which isgiven in FIG. 29 b further illustrates how the multi-coil partition canreduce the eddy current loss.

As FIG. 29 b illustrates, the single coil trace 29-7 is broken up intoindividual coils 29-8 separated by spaces 29-9. As pointed outpreviously, if the magnetic coupling is large between the coils, theparallel combination of the individual coils maintains a value of theinitial inductance but decreases the overall parasitic resistance ofeach individual coil. In addition, because the width of the coil hasdecreased, the possibility of forming eddy currents is decreased helpingto eliminate this portion of the loos. Assume for example, that thewidth of the space 29-9 is a micron or less while the width of the traceis on the order of a micron or more. These width dimension may beadjusted dependant on the electro-migration consideration. Thisstructure causes the size of the eddy current loop to be limited to thewidth of the coil. However, the overall inductance can be better thanthat of the conventional single coil with a parasitic resistance that issimilar to the single coil. Thus, the multi-transformer inventivetechnique used in horizontal planar inductors offers a decrease in eddycurrent loss.

FIG. 29 c illustrates a magnified version of the region 29-10 in FIG. 29b that indicates the sidewall capacitance 29-11 between two coils 29-8.These capacitances could be quite large particularly due to the aspectratio of the metal thickness. However, since all the coils are inparallel, the potential difference across each of the capacitors willremain constant. Thus, the structure of multi-coils transformerseliminates the concern of sidewall capacitance.

FIG. 30 a depicts a multi-coil transformer formed from a vertical planarinductor structure. This structure is similar to that given in FIG. 21 aexcept that the connections at the ingress and egress ports aredifferent. In this connection shown in FIG. 30 a there are two sets ofhelix coils. The first helix is formed using the bottom two coils 30-5and 30-4 as can be determine by tracing from the left port carrying thecurrent 30-3 into the ingress port of the coil 30-5. The egress port ofcoil 30-5 is shorted to the ingress port of coil 30-4 and then theegress port of coil 30-4 is connected to the port carrying the current30-9. The second helix follows a similar trajectory in the top two coils30-3 and 30-2. Furthermore, note that both helixes are connected inparallel. That is, the left port carrying current 30-8 connects to theingress ports of coils 30-3 and 30-5. The right port that carries thereturn current 30-9 connects to the egress ports of coils 30-2 and 30-4.This type of structure offers the ability to increase the inductancesince N=2, and decrease the resistance since two helixes are inparallel. Furthermore, each of these coils can be segregated into manysegmented coils (as shown in FIG. 29 b) to decrease the eddy currentlosses.

A circuit representation of the structure in FIG. 30 a is given in FIG.30 b. Assume that all inductances are equal to L. The coils; 30-2, 30-3,30-4 and 30-5 are represented by the inductances 30-10, 30-11, 30-12 and30-13, respectively. The first helix formed by the coils 30-4 and 30-5are represented by the inductors 30-12 and 30-13. Note that the twoinductors 30-12 and 30-13 are in series. The inductors 30-10 and 30-12form the second helix where both of these inductors are in series aswell. Finally, both of these helixes are connected in parallel to lowertheir effective resistance but still maintaining the higher inductancebecause of the mutual magnetic coupling.

Breaking up a wire into many parallel runners to help reduce eddycurrent loss is show in FIG. 31. The multi-coil runner 31-5 crosses fromtop to bottom and crosses over the bridge 31-4 which is part of thecross-under 31-1. The bridge 31-4 is used to connect the multi coilrunner from the left 31-2 to the multi coil runner on right 31-7. Theparallel coils 31-2 are combined into one solid unit 31-3. Vias connectthe solid unit 31-3 to the bridge 31-4, then vias are again used toconnect the bridge 31-4 to the solid unit 31-6 of the sets of parallelcoils 31-7 located on the right. Thus, whenever a multi-coil runnermakes a connection to another metal layer, the runner are combined intoone solid unit for easy access to vias.

FIG. 32 illustrates a MCM (Multi-Chip Module) 32-1 containing aninductor on each of the two die making up the MCM. The cross-sectionalview has been simplified to provide the crux of the idea. For example,only one metal layer is shown on each die but those skilled in the artwill appreciate that additional metal and dielectric layers can be addedto the diagram without altering the idea. The lower die contains asubstrate 32-2 and a dielectric layer 32-3 has been deposited on thesubstrate. A metal layer 32-5 with the shape of a coil (not shown) hasbeen patterned on top of the dielectric layer 32-3. The coil 32-5 hasits first lead electrically connected to a via and a metal layer 32-6.The second lead of the coil is electrically connected to the via and ametal layer 32-9. The solder bumps 32-7 connect the lower die to theupper die.

The upper die has a similar structure as the lower die to simplify thedescription and many of the numerals describing the features are thesame. A dielectric layer 32-3 is deposited on the substrate 32-2. Ametal layer 32-8 with the shape of a coil (not shown) has been patternedon top of the dielectric layer 32-3. The coil 32-8 has its first leadelectrically connected to a first via and a metal layer 32-6. The secondlead of the coil is electrically connected to a second via and a metallayer 32-9. The solder bumps 32-7 not only provide mechanical support tothe two die but electrically connect the two coils in parallel as well.These two coils are now electrically connected in parallel and aremagnetically coupled due to their proximity to each other.

Finally, it is understood that the above description are onlyillustrative of the principle of the current invention. In accordancewith these principles, those skilled in the art may devise numerousmodifications without departing from the spirit and scope of theinvention. For example, the multi-coil transformer element can beutilized in various circuits such as filters, antennas, and other RFcircuits. In another example, the MOS devices illustrated in theregenerative circuit can be replaced by BJT device to provide a negativeimpedance and maintain the oscillations.

1. A RLC tank circuit comprising; a first lead and a second lead, atleast one capacitor coupled to the first lead and the second lead, twoor more elements each coupled to the first lead and the second leadwhere each element comprises; an inductor with a parasitic resistance.2. The circuit of claim 1 wherein at least two of the inductors aremagnetically coupled to each other.
 3. The circuit of claim 1 where thesecond lead is a power supply lead.
 4. The circuit of claim 1 whereinthe capacitor has a parasitic resistance.
 5. The circuit of claim 1,comprising; a regenerative circuit coupled to the first lead and thesecond lead.
 6. The circuit of claim 5 wherein at least one of the leadsdrives an external capacitive load.
 7. The circuit of claim 5 whereinthe regenerative circuit is comprised of devices selected from the groupconsisting of MOS transistors, CMOS transistors, and BJT transistors. 8.The circuit of claim 5 comprising an adjustable capacitor selected fromthe group consisting of a voltage controlled varactor,digitally-weighted gate capacitance, voltage controllable MOS depletionlayer capacitance, and voltage controllable MOS enhancement layercapacitance.
 9. A RLC tank circuit comprising; a first lead, a secondlead and a power supply lead, a capacitor coupled to the first lead andthe power supply lead, a second capacitor coupled to the second lead andthe power supply lead, and two or more elements each coupled to thefirst lead and the second lead where each element comprises; an inductorwith a parasitic resistance.
 10. The circuit of claim 8 wherein at leasttwo of the inductors are magnetically coupled to each other.
 11. Thecircuit of claim 8 where the second lead is a power supply lead.
 12. Thecircuit of claim 8 wherein the capacitor has a parasitic resistance. 13.The circuit of claim 8, comprising; a regenerative circuit coupled tothe first lead and the second lead.
 14. The circuit of claim 12 whereinat least one of the leads drives an external capacitive load.
 15. Thecircuit of claim 13 wherein the regenerative circuit is comprised ofdevices selected from the group consisting of MOS transistors, CMOStransistors, and BJT transistors.
 16. The circuit of claim 13 comprisingan adjustable capacitor selected from the group consisting of voltagecontrolled varactors, digitally weighted gates capacitances, voltagecontrollable MOS depletion layer capacitance, and voltage controllableMOS enhancement layer capacitance.
 17. An apparatus comprising; aregenerative circuit with a first lead and a second lead, and a tankcircuit comprising; at least one capacitor, and at least two inductorsconnected in parallel, wherein the tank circuit is coupled to the firstlead and second lead.
 18. The apparatus of claim 17 wherein at least twoof the inductors are magnetically coupled to each other.
 19. Theapparatus of claim 17 wherein each inductor has a parasitic resistance.20. An apparatus comprising; a tank circuit comprising; at least onecapacitor, where the one capacitor is connected in parallel to a firstinductor, a regenerative circuit coupled to the tank circuit, and atleast one inductor connected in parallel to the first inductor.
 21. Theapparatus of claim 20 wherein at least two of the inductors aremagnetically coupled to each other.
 22. The apparatus of claim 20wherein each inductor has a parasitic resistance.
 23. A method ofdecreasing the parasitic resistance of a tank circuit comprising thesteps of; forming a tank circuit using at least one capacitor and afirst inductor having a first parasitic resistance, placing at least oneadditional inductor having a second parasitic resistance in parallel tothe first inductor, reducing an overall parasitic resistance of theparallel combination of the inductors due to the parallel combinationresistance rule, thereby decreasing the parasitic resistance within thetank circuit.
 24. A method of improving the Q of a tank circuitcomprising the steps of; forming a tank circuit using at least onecapacitor and a first inductor having a first parasitic resistance,placing at least one additional inductor having a second parasiticresistance in parallel to the first inductor, reducing an overallparasitic resistance of the parallel combination of the inductors due tothe parallel combination resistance rule, reducing an overall inductanceof the parallel combination of the inductors due to the parallelcombination inductance rule, forming a magnetic coupling between atleast two of the inductors and increasing the mutual coupling of theoverall inductance, thereby improving the Q of a tank circuit.
 25. Anapparatus comprising; a capacitor with a first node and a second node, atransformer comprising; a first coil with a first lead and a secondlead, and a second coil with a third lead and a fourth lead, wherein thefirst node is connected to both the first lead and the third lead, andthe second node is connected to both the second lead and the fourthlead.
 26. The apparatus of claim 25 wherein the first and second coilshave a parasitic resistance.
 27. The apparatus of claim 25 comprising aregenerative circuit coupled to the first and second nodes.
 28. Anoscillator circuit comprising; a regenerative circuit with at least twonodes, and at least one transformer with two or more leads, wherein eachnode of the regenerative circuit is connected to at least one lead ofthe transformer.
 29. An apparatus for generating oscillating signalscomprising; a regenerative circuit with a first and second nodes, atransformer comprising; a first coil with first and second leads havinga first inductance, a second coil with third and fourth leads having asecond inductance, and a magnetic coupling between the primary andsecondary coils, whereby the first node is connected to the first andfourth leads, and the second node is connected to the second and thirdleads, such that the magnetic coupling causes the transformer to presentan effective inductance to the regenerative circuit that is differentthan the parallel combination of the first and second inductances. 30.The apparatus of claim 29 were the effective inductance is less than theparallel combination of the first and second inductances.
 31. Theapparatus of claim 29 were the effective inductance is more than theparallel combination of the first and second inductances.
 32. Anoscillator comprising; at least one capacitor, at least one inductorwith a parasitic resistance, means for forming a tank circuit using theat least one inductor and at least one capacitor, means for connecting aregenerative circuit to the tank circuit, an addition inductor that isplaced in parallel to the at least one inductor, whereby an overallparasitic resistance is decreased in the oscillator.